As described in Examined Patent Publication (Kokoku) No. 7-13841 and Unexamined Patent Publication (Kokai) No. 8-96355, a magnetic recording medium obtained by subjecting a substrate made of an aluminum alloy to a non-magnetic plating treatment such as an alumite treatment, Ni—P plating or the like, coating with a primary layer of Cr, coating with a magnetic thin film layer of a Co-based alloy, and then coating with a carbonaceous protective film has hitherto been used exclusively as a magnetic recording medium.
Such a magnetic recording medium (magnetic disc) has a high dimensional accuracy and is liable to exhibit an enhanced recording density because of its hard substrate. When a magnetic head is contacted with a magnetic disk rotating at high speed, there arises a so-called head crushing to break down the magnetic head. To avoid head crushing, the magnetic head is employed in levitated state.
However, with the increase of the density of the magnetic disk, the levitation height of the head decreases more and more and levitation height of not more than 1.5 μm has recently been required. It is the greatest point for improvement in recording density to maintain the levitation height of the head at a very small value, and it is the greatest technical object to prevent head crushing while maintaining a low levitation height.
Studying about the magnetic disk, absorption sometimes arises between the levitated surface of the magnetic head and the magnetic disk when the disk is at a standstill in the case of writing on the magnetic disk or reproduction.
The adsorption phenomenon described above is caused by the fact that, when the levitated surface of the magnetic head and the surface of the magnetic disk are very smooth and mutually face a microspace, the microspace is filled with molecules such as O2, N2, H2O, etc. and a large adsorptivity is caused by a surface tension. Such an adsorption phenomenon causes such a disadvantage that a large amount of electric power is consumed on the starting of a motor.
To prevent such an adsorption phenomenon, a substrate is textured, that is, the surface of the substrate is first mirror-finished and then the surface is adjusted to a proper surface roughness by roughening before forming a magnetic film on the substrate.
As the texturing method, the following method has hitherto been employed. For example, a so-called textured substrate can be obtained by directly transferring a Ni—P plated substrate in a radial direction of the substrate while pressing a polishing tape comprising deposited abrasive grains made of silicon carbide, alumina or diamond from the back surface of the tape by means of a roll in the state where the substrate is rotated, thus forming concentric streaks on the surface of the substrate.
However, it is considerably difficult to roughen the surface of the substrate by adjusting to a proper surface roughness using a conventional polishing tape, thus causing a problem that the surface of the substrate is too roughened.
To solve such a problem, Unexamined Patent Publication (Kokai) NO. 6-295432 has suggested that a constituent fiber of a polishing tape used for texturing is made thin as compared with a conventional one and the surface roughness Ra is adjusted to 40 Å or less by using a polishing tape composed of fibers having a diameter of 5 μm (fineness: 0.1 denier).
Also Unexamined Patent Publication (Kokai) No. 8-96355 has suggested a nonwoven fabric wherein a water retaining rate is not less than 400% and a fiber strength is not more than 11 (kg/5 cm width) in 10% modulus strength (longitudinal direction) when drying and, moreover, a difference between 10% modulus strength (longitudinal direction) when drying and that when wetting is not more than 8 (kg/5 cm width).
The respective polishing tapes described in the publications described above are provided with a characteristic constitution, thereby exerting a peculiar effect. According to a trend for several years ahead (five to six years), fixed abrasive grains have changed gradually into free abrasive grains in a polishing system, whereby a raised tape as a raw tape material has changed into a flocky tape and, moreover, a fabric and a nonwoven fabric as the other raw material has changed into a raw material with a more soft surface. Therefore, these polishing tapes are exclusively used at the present.
In a conventional polishing system, a tape using an abrasive having a large grain size and a fiber having a large single yarn fineness was satisfactorily employed. However, with the increase in the requirement of information density on the substrate, it has been required to develop a polishing tape using a microfiber having a small single yarn fineness, which can sufficiently retain an abrasive having a very small grain size.
In light of circumstances described above, the present invention has been made and an object thereof is to provide a novel polishing tape which has a polished substrate having a small surface roughness as compared with a conventionally suggested polishing tape and prevents the occurrence of crushing to the utmost, and is also capable of noticeably improving levitation characteristics of the magnetic head.